Add revspec documentation for ':path', ':[0-3]:path' and git-describe

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-rev-parse.txt

@@ -111,14 +111,19 @@ SPECIFYING REVISIONS
 
 A revision parameter typically, but not necessarily, names a
 commit object.  They use what is called an 'extended SHA1'
-syntax.
+syntax.  Here are various ways to spell object names.  The
+ones listed near the end of this list are to name trees and
+blobs contained in a commit.
 
 * The full SHA1 object name (40-byte hexadecimal string), or
   a substring of such that is unique within the repository.
   E.g. dae86e1950b1277e545cee180551750029cfe735 and dae86e both
   name the same commit object if there are no other object in
   your repository whose object name starts with dae86e.
 
+* An output from `git-describe`; i.e. a closest tag, followed by a
+  dash, a 'g', and an abbreviated object name.
+
 * A symbolic ref name.  E.g. 'master' typically means the commit
   object referenced by $GIT_DIR/refs/heads/master.  If you
   happen to have both heads/master and tags/master, you can
@@ -156,6 +161,15 @@ syntax.
   and dereference the tag recursively until a non-tag object is
   found.
 
+* A suffix ':' followed by a path; this names the blob or tree
+  at the given path in the tree-ish object named by the part
+  before the colon.
+
+* A colon, optionally followed by a stage number (0 to 3) and a
+  colon, followed by a path; this names a blob object in the
+  index at the given path.  Missing stage number (and the colon
+  that follows it) names an stage 0 entry.
+
 Here is an illustration, by Jon Loeliger.  Both node B and C are
 a commit parents of commit node A.  Parent commits are ordered
 left-to-right.